In recent years, in the automotive field, due to the growing awareness of environmental issues, exhaust gas regulations have become more stringent and countermeasures have been underway to suppress carbon dioxide gas exhaust. In addition to countermeasures on fuel such as bioethanol or biodiesel fuel, the countermeasures, such as further weight reduction or the installation of exhaust gas treatment devices such as an Exhaust Gas Recirculation (EGR), a Diesel Particulate Filter (DPF), and a urea Selective Catalytic Reduction (SCR) system, have been in practice. Further, in order to improve fuel efficiency, an exhaust heat recovery device which thermally recovers exhaust heat has also come to be mounted.
In the EGR cooler, exhaust gas from an engine is cooled by using engine cooling water and then the exhaust gas is returned to an intake side and is combusted again. Thus, the combustion temperature is lowered and an amount of NOx which is a poisonous gas is reduced. In addition, the exhaust heat recovery device is a system where exhaust gas heats engine cooling water and the heated engine cooling water is used for a heater or the warm-up of an engine, and is also called as an exhaust heat recirculation system. Accordingly, the exhaust heat recovery device shortens a time from cold start to engine stop in hybrid vehicles, and contributes to improvement in fuel efficiency particularly in the winter.
Further, in the field of water heaters (hot water supplies), with the spread of eco-friendly devices, heat exchangers have been widely applied. In a hot water supply by gas, a latent heat recovery type hot water supply by gas has become widespread which further includes a stainless steel secondary heat exchanger so as to recover latent heat from a high temperature exhaust gas of approximately 150° C. to 200° C. which was exhausted without recovering in the hot water supply by gas of the related art. Further, a hot water supply by electricity of the related art included a heater. However, the hot water supply by electricity has been changed to a CO2 refrigerant heat pump type hot water supply, which is commonly known as EcoCute (registered trademark), capable of reducing electrical energy to ⅓ or lower and in the CO2 refrigerant heat pump type hot water supply, a heat exchanger is also used.
Such a heat exchanger is required to have a good thermal conductivity to obtain good thermal efficiency. In addition, since the heat exchanger is in contact with exhaust gas, the heat exchanger is required to have excellent corrosion resistance to exhaust gas condensate water. In the case of automobile parts, an EGR cooler or an exhaust heat recovery device which has a possibility that corrosion may cause a serious accident such as the leakage of cooling water is required to have greater safety and further excellent corrosion resistance. In addition, since the structure of a heat exchange section is complicated, the heat exchange section is assembled not only by welding but also by brazing. The material for the heat exchange section assembled by brazing is required to have good brazeability.
As the material used for a heat exchanger, generally, austenitic stainless steel such as SUS304 and SUS316L is used. However, from the viewpoint of thermal conductivity, intergranular corrosion resistance and stress corrosion cracking resistance, ferritic stainless steel has attracted attention.
In Patent Document 1, a ferritic stainless steel is disclosed which includes C: 0.01% or less, Cr: 10.5% to 13.5%, N: 0.05% or less, and at least one of Ti, Nb and Ta and is easily brazed by a brazing filler material. Here, wettability is affected by the amount of Ti and thus the amount of Ti is limited to 0.12% or less. From the viewpoint of brazing, it is desirable not to add Ti.
In Patent Document 2, a ferritic stainless steel having excellent brazeability is disclosed which includes C: 0.03% or less, Si: 0.02% to 1.5%, Mn: 0.02% to 2%, Cr: 10% to 22%, Nb: 0.03% to 1%, Al: 0.5% or less, and N: 0.05% or less. In Patent Document 2, brazeability is ensured by limiting the amounts of Ti and Al based on the relational expression of Ti, N and Al.
In Patent Document 3, a ferritic stainless steel for brazing is disclosed which includes C: 0.03% or less, Si: more than 0.1% to 1% or less, Mn: 2% or less, P: 0.05% or less, S: 0.03% or less, Cr: 16% to 25%, Nb: 0.15% to 0.8%, Ti: 0.03% or less, Al: 0.03% or less, and N: 0.03% or less. In Patent Document 3, particularly, brazeability in a hydrogen atmosphere is ensured by more strictly limiting the amount of Al added in addition to Ti.
Since a heat exchanger provided in an EGR cooler or an exhaust heat recovery device is required to have excellent corrosion resistance, a ferritic stainless steel having a large amount of Cr is generally applied. Stainless steel exhibits corrosion resistance by forming a passivation film which is rich in Cr in the surface. As the amount of Cr in the material increases, a film having a higher degree of protection is formed. On the other hand, it is required to temporarily reduce and remove the passivation film at the time of brazing. However, in the related art, there has been no consideration of the reduction properties of a passivation film, which is formed in stainless steel, particularly, stainless steel having a high Cr content.
In addition, in order to cope with exhaust gas regulations and fuel efficiency regulations, which have become tighter every year, countermeasures to cope with the regulations have been made even in the automotive field. As an example, direct engine injection may be mentioned. When direct engine injection is achieved, it is possible to reduce fuel consumption and to improve output at the same time. Also, exhaust gas can be reduced. In addition, due to high compatibility of the direct injection engine with a supercharger, even when the direct engine injection is combined with a downsized engine, power performance can be maintained.
In the direct injection engine, fuel discharged from a fuel tank is pressurized by a pump and supplied to the engine through a delivery pipe or the like. Since the pressurized fuel is intermittently injected into the engine, the pressure of the fuel is easily changed. Therefore, a pressure adjusting part is required in some cases. Since a fuel supply system part such as a pressure adjusting part is disposed in the proximity of the engine, the temperature of the part is easily increased. Therefore, the material used for the fuel supply system part is required to have strength. In order to ensure strength, it is considered to increase the thickness of the material. However, when the thickness of the material is increased, the weight of the material is also increased, which leads to an increase in fuel consumption.
On the other hand, from the viewpoint of suppressing carbon dioxide gas exhaust, biofuels such as bioethanol and biodiesel fuel have been widely used. For example, bioethanol which is included in biofuel is a factor which causes aluminum to be corroded. Accordingly, the material used for the fuel supply system part is required to have good corrosion resistance to biofuel. From the viewpoint of the above-described strength and corrosion resistance, stainless steel has attracted attention as the material for the fuel supply system part.
In addition, most fuel supply system parts such as a delivery pipe and the like have complicated structures. Therefore, members are assembled by brazing in many cases. Accordingly, the material used for the fuel supply system part is required to have not only good strength and corrosion resistance but also good brazeability.
In Patent Document 4, a fuel supply apparatus is disclosed which is capable of damping fuel pressure pulsation in a delivery pipe from low fuel pressure operation time to high fuel pressure operation time. In Patent Document 4, it is disclosed that stainless steel can be used for a pulsation damping pipe. However, the details of the stainless steel material used are not described.
In Patent Document 5, a high pressure fuel delivery pipe for a direct injection engine is disclosed which is capable of preventing a leakage of a high pressure fuel from a joint surface between a mounting stay and a body pipe and improving bonding strength of the mounting stay. It is disclosed that this delivery pipe is produced by brazing stainless steel. However, in Patent Document 5, the details of the stainless steel used are not described.
In Patent Document 6, a ferritic stainless steel sheet is disclosed which contains, by mass %, C: ≤0.01%, Si: ≤1.0%, Mn: ≤1.5%, P: ≤0.06%, S: ≤0.03%, Cr: 11% to 23%, Ni: ≤2.0%, Mo: 0.5% to 3.0%, Al: ≤1.0%, and N: ≤0.04% so as to satisfy a relational expression of Cr+3.3Mo≥18, and further contains either one or both of Nb: ≤0.8% and Ti: ≤1.0% so as to satisfy a relational expression of 18≤Nb/(C+N)+2Ti/(C+N)≤60, in which the grain size number of ferrite grains is 6.0 or more and the average r value is 2.0 or more.
In Patent Document 7, a ferritic stainless steel sheet is disclosed which contains, by mass %, C: ≤0.01%, Si: ≤1.0%, Mn: ≤1.5%, P: ≤0.06%, S: ≤0.03%, Al: ≤1.0%, Cr: 11% to 20%, Ni: ≤2.0%, Mo: 0.5% to 3.0%, V: 0.02% to 1.0%, and N: ≤0.04%, and further contains either one or both of Nb: 0.01% to 0.8% and Ti: 0.01% to 1.0%, in which the height of an undulation generated on the surface of the steel sheet is 50 μm or less when the steel sheet is subjected to a uniaxial tension and deformed by 25%. The technologies in both Patent Documents 6 and 7 deal with corrosion resistance to typical gasoline and there is no description of corrosion resistance to biofuel.
A material used for an automotive fuel supply system part, particularly, a direct injection engine fuel supply system part is required to mainly have strength, corrosion resistance and brazeability. Stainless steel exhibits corrosion resistance by forming a passivation film which is rich in Cr in the surface and as the Cr content in the material increases, a film having a higher degree of protection is formed. Thus, excellent corrosion resistance is exhibited. On the other hand, at the time of brazing of stainless steel, it is required to temporarily reduce and remove this passivation film. However, in the related art, there has been no consideration of the reduction properties of a passivation film, which is formed in stainless steel, particularly, stainless steel having a high Cr content. In the stainless steel having a high Cr content, the reduction resistance of the passivation film is high. In the related art, stainless steel having both good corrosion resistance and good brazeability has not been suggested.